1. Field of the Invention
This invention relates to optical pickups. More particularly, the invention relates to an optical pickup used to read information out of an optical disc (hereinafter referred merely to as "disc") such as CD and DVD.
2. Description of the Prior Art
There is one example of an optical pickup adapted to read information out of a disc as disclosed in Japanese Patent Publication No. H5-9851 [G11B7/09, G02B7/28, G11B7/135]. The optical pickup of this kind is required to have functions to detect focusing and tracking errors, in addition to the function to reproduce record information. Conventionally, the well-known Foucault's method, astigmatic method and the like have been utilized to detect focusing errors while the push-pull method, three-beam method and the like have been used to detect tracking errors. Where using the Foucault.times.s method and the three-beam method in combination, a photo-detector 1 has been used which includes light receiving elements 2a-2c for receiving a main beam and light receiving elements 3a and 3b for receiving sub-beams. Due to this, a focusing error signal is determined from a difference (S2a-S2b) of between output signals of the light receiving elements 2a and 2b, while a tracking error signal is determined by a difference (S3a-S3b) between output signals of the light receiving elements 3a and 3b.
In general, the reflected light sub-beam from the disc travels through the lens in a somewhat oblique fashion. Accordingly, the two sub-beams separated by a diffraction elements 4 are not even but different in spot size. That is, the separated two beams include one beam greater in spot size (hereinafter referred to as "light-shield") and another beam smaller in spot size (hereinafter referred to as "aperture"). Due to this, the beam portion diffracted as an aperture side and the beam portion diffracted as a light-shield side will not constitute symmetry on a light receiving surface of the photo-detector 1. On the light receiving surface, the beam B as a light-shield side is greater in spot size than the beam A as an aperture side.
In the photo-detector 1 for a conventional optical pickup, such a phenomenon as stated above is not taken into consideration in design. That is, the light receiving elements 3a, 3b are each formed in a constant width throughout its entire length, with the result that the beam B on the light-shield side tends to go out of the light receiving elements 3a, 3b as shown in FIG. 13. Due to this, there has been a problem that tracking error balance or jitter is greatly varied by change of temperature, resulting in instability in optical pickup characteristics.
Meanwhile, when adjusting the positional relationship between the diffraction element 4 and the light receiving element 1 in the optical pickup of this kind, focusing servo is effected by employing a servo circuit while actually rotating the disc to displace the diffraction element 4 such that jitter values observed by a jitter meter exhibit a best. Due to this, there has been a problem that operation of adjustment is troublesome and time consuming. Further, the requirement of a servo circuit leads to increase in cost. In addition, there has also been a problem that the optical pickup is unstable in characteristic because the diffraction element is displaced by using as a reference a jitter value that is liable to suffer from noises.